## What changes were proposed in this pull request?
The result of the `Last` function can be wrong when the last partition processed is empty. It can return `null` instead of the expected value. For example, this can happen when we process partitions in the following order:
```
- Partition 1 [Row1, Row2]
- Partition 2 [Row3]
- Partition 3 []
```
In this case the `Last` function will currently return a null, instead of the value of `Row3`.

This PR fixes this by adding a `valueSet` flag to the `Last` function.

## How was this patch tested?
We only used end to end tests for `DeclarativeAggregateFunction`s. I have added an evaluator for these functions so we can tests them in catalyst. I have added a `LastTestSuite` to test the `Last` aggregate function.

Author: Herman van Hovell <hvanhovell@databricks.com>

Closes #15348 from hvanhovell/SPARK-17758.